Nickel(II) and cobalt(II) complexes of potentially quinquedentate macrobicyclic ligands. Reversible binding of dioxygen to a cobalt(II) complex

Two new macrobicyclic ligands which are potentially quinquedentate have been prepared and characterised. Variable-temperature NMR studies show that the nickel(II) complex of one of these ligands exists as a mixture of two conformers at low temperature which take part in two independent exchange processes as the temperature is raised. In the first, one conformer undergoes a form of self exchange and in the second, the two conformers interconvert. The ESR spectrum of the cobalt(II) complex of one of the new ligands indicates that it exists as a mixture of four- and five-co-ordinate species, with the five-co-ordinate material arising from intermolecular binding of the ligand secondary amine group to the cobalt(II) ion. This complex binds dioxygen reversibly and the O₂ adduct has reasonable stability with respect to autoxidation.